OSL script includes and reuse
After some experimentation, it appears that #include-ing OSL files works with external scripts, but not internal.

The OSL compiler takes a shader file as input, and seems to use the directory of that file as a base for further includes. It also looks in the \scripts\addons\cycles\shader directory.

If external scripts are used, the OSL compiler therefore looks in the correct directory, and will find included files correctly.

If internal scripts are used, the script being compiled is moved by itself to a temporary directory, given a random name, and deleted afterwards. This means that internal scripts cannot use #includes unless the included files are placed in the \scripts\addons\cycles\shader directory.